BIG dog park in Middletown...just off of Levels rd off of rt301 (south of Summit bridge approx 10mins. from Glasgow/peoples plaza area) two LARGE fenced in bark parks with plenty of room to run and play.
Also Iron Hill park has 2 small bark parks...Iron Hill park is located just off of Welsch Track road which is off of Old Baltimore Pike from 896 near I95....

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Sandi W. I think Carousel Park in Pike Creek has a bark park area. Also Lums Pond has a dog area where they can run free....and swim too.

Laurel H. The one at Carousel has a pond and a large unfenced area for running/playing. Use the entrance that's on Skyline Drive, just off Linden Hill Road and between the intersection (Linden Hill/Skyline Drive) and Skyline Middle School. Look for the white post/rail fence and gate. (You can go in the main entrance, but it's quite a schlep to the bark park.)

There's a large fenced-in dog park (with areas for both large and small dogs) in Iron Hill Park (off of Old Baltimore Pike to the west of 896). You have to follow the access road all the way to the back and it's behind the playground. I've been there once during the week and it was empty, but it's supposedly more active on weekends.

There's also an off-leash dog section of Lums Pond State Park. It's marked on the map that's on the official state website for the park. I haven't been there yet.